How to Use Once and OnceLock for One-Time Initialization

Crates/Once Cell

2026-04-17T12:05:42+00:00

Use OnceLock for safe, thread-safe one-time initialization that returns a reference to the initialized value.

Use Once for single-threaded one-time initialization and OnceLock for thread-safe initialization that returns a reference to the value. OnceLock is the modern, preferred choice for most use cases as it handles both single and multi-threaded scenarios safely.

use std::sync::OnceLock;

static CONFIG: OnceLock<String> = OnceLock::new();

fn get_config() -> &'static str {
    CONFIG.get_or_init(|| {
        // Expensive initialization runs exactly once
        String::from("loaded configuration")
    })
}